Murphy’s 7 laws of industrial wireless communications

Applying wireless technologies for industrial communications doesn’t have to be as tough as Murphy’s Law (if anything can go wrong, it will), if you remember these Murphy’s 7 laws of industrial wireless communications. Wireless troubleshooting tips follow, including the number-one cause of wireless woes.

Mike Fahrion, B&B Electronics

10/18/2010

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Applying wireless technologies for industrial communications doesn’t have to be as tough as Murphy’s Law (if anything can go wrong, it will), as long as you remember these Murphy’s 7 laws of industrial wireless communications. Do you know the number-one cause of wireless woes? That’s included below.

1. You can’t trump the laws of physics. Stick with the lowest frequency possible.

Even Murphy knows better than to argue with physics. Industrial applications typically operate in “license free” ISM frequency bands that vary from country to country. The most common frequencies are:

2.4 GHz—nearly worldwide;

915 MHz band—North America, South America, some other countries; and

868 MHz band—Europe.

Radio frequency (RF) power is measured in milli Watts (milliwatts; mW) or in a logarithmic scale of decibels (dB), or decibels referenced to 1 mW of power (dBm). Since RF power attenuates as a logarithmic function, the dBm scale is most useful. These scales relate as follows:

1 mW = 0 dBm

A 2-fold increase in power yields 3 dB of signal.

2 mW = 3 dBm

A 10-fold increase in power yields 10 dB of signal.

4 mW = 6 dBm

A 100-fold increase in power yields 20 dB of signal.

10 mW = 10 dBm

100 mW = 20 dBm

1 W = 30 dBm

As frequency rises, available bandwidth typically rises, but then distance and ability to overcome obstacles is reduced. For any given distance, a 2.4 GHz installation will have roughly 8.5 dB of additional path loss when compared to 900 MHz. However, lower frequencies require larger antennas to achieve the same gain.

Before you lift a finger toward the perfect wireless installation, think about the impact of wireless communications on your application. Acceptable bit error rates are orders of magnitude higher than wired communications. Most radios quietly handle error detection and retries for you at the expense of throughput and variable latencies.

Software must be well-designed and communication protocols must be tolerant of variable latencies. Not every protocol can tolerate simply replacing wires with radios. Protocols sensitive to inter-byte delays may require special attention or specific protocol support from the radio. Do your homework up front to confirm that your software won’t choke, that the intended radio is friendly toward your protocol, and that your application software can handle it as well.

3. Transmit power does not necessarily equal long-range performance. You must know the receive sensitivity.

The more sensitive the radio, the lower the power signal it can successfully receive, stretching right down to the noise floor. There is such variety in “specsmanship” for radio sensitivity that it can be difficult to compare products meaningfully. Unless you’re in a high RF noise environment, the odds are good that the noise floor will be well below the receive sensitivity, so the manufacturer’s rated receive sensitivity will be a key factor in your wireless system and range estimates.

You can often improve your receive sensitivity, and therefore your range, by reducing data rates over the air. Many radios give the user the ability to reduce the communication rate to maximize range. Also, receive sensitivity improves at lower frequencies, giving 900 MHz radios a significant range advantage.

Contrary to popular opinion, no black art is required to make a reasonable prediction of the range of a given radio signal.

Most parameters above are easily gleaned from the wireless device manufacturer’s data. That leaves only path loss and, in cases of heavy RF interference, RF noise floor as the two parameters you must establish for your installation.

RF background noise comes from many sources, ranging from solar activity to high-frequency digital products to all forms of other radio communications. That background noise establishes a noise floor, the point where the desired signals are lost in the background ruckus.

If your environment has high degrees of RF noise in your frequency band, then use the noise floor figures instead of radio receive sensitivity in your calculations. When in doubt, look around. Antennas are everywhere nowadays—on the sides of buildings, water towers, billboards, chimneys, and even disguised as trees. Interference may not be obvious.

Fade margin is a term critical to wireless success. Fade margin describes how many dB a received signal may be reduced without causing system performance to fall below an acceptable value. Walking away from a newly commissioned wireless installation without understanding how much fade margin exists is the number-one cause of wireless woes.

Establishing a fade margin of no less than 10dB in good weather conditions virtually ensures that the system will continue to operate effectively in a variety of weather, solar, and RF interference conditions. Outside conditions also require a cabinet for protection or select IP67 outdoor-rated wireless units (such as B&B Electronics’ Zlinx Xtreme radios and I/O modules; see photos) to handle Mother Nature. (See the next law about wireless signal obstacles; metal is not your friend.)

6. Only an eternal optimist would ever attempt a system at the manufacturer’s maximum advertised distance. Remember the real world: Stay clear of obstacles and maintain line of sight.

In a clear path through the air, radio signals attenuate with the square of distance. Doubling range requires a four-fold increase in power, therefore:

Halving the distance decreases path loss by 6dB.

Doubling the distance increases path loss by 6dB.

When indoors, paths tend to be more complex, so use a more aggressive rule of thumb, as follows:

Halving the distance decreases path loss by 9dB.

Doubling the distance increases path loss by 9dB.

Radio manufacturers advertise “line of sight” range figures. Line of sight means that, from antenna A, you can see antenna B. For every obstacle in the path, de-rate the “line of sight” figure specified for each obstacle in the path. The type, location, and number of obstacles will all impact path loss. Obstructions located close to the antennas will cause the most dramatic loss.

Don’t underestimate the distance between antennas. If it’s a short-range application you can pace it off. If it’s a long-range application, increase distance accuracy quickly by using a global positioning system (GPS) or Google Maps.

The most effective way to reduce path loss is to elevate the antennas. At approximately 6 feet high (2 m), line of sight due to the Earth’s curvature is about 3 miles (5 km), so anything taller than a well-manicured lawn becomes an obstacle.

Industrial installations often include many reflective obstacles leading to numerous paths between the antennas. The received signal is the vector sum of each of these paths. In multiple-path environments, simply moving the antenna slightly can significantly change the signal strength.

Some obstacles are mobile. More than one wireless application has been stymied by temporary obstacles such as a stack of containers, a parked truck or material handling equipment. Remember, metal is not your friend. An antenna will not transmit out from inside a metal box or through a storage tank.

Path loss rules of thumb:

To ensure basic fade margin in a perfect line of sight application, never exceed 50% of the manufacturer’s rated line of sight distance. This in itself yields a theoretical 6dB fade margin, still short of the required 10dB.

De-rate more aggressively if you have obstacles between the two antennas, but not near the antennas.

De-rate to 10% of the manufacturer’s line of sight ratings if you have multiple obstacles, obstacles located near the antennas, or the antennas are located indoors.

7. It’s not all about the radio; use the wrong antenna or cable, and you’re toast.

Antennas increase the effective power by focusing the radiated energy in the desired direction. Using the correct antenna not only focuses power into the desired area but also reduces the amount of power broadcast into areas where it is not needed.

If your job site is already bristling with other antennas, try to separate yours as much as possible. Most antennas broadcast in a horizontal pattern, so vertical separation is more meaningful than horizontal. Try to separate antennas by a minimum of two wavelengths, 26 inches (0.66 m) at 900 MHz, or 10 inches (0.25 m) at 2.4 GHz.

Use high-quality RF cable between the antenna connector and your antenna, and ensure that all connectors are high-quality and carefully installed to help signal propagation. Factor in a 0.2 dB loss per coaxial connector in addition to the cable attenuation itself. Typical attenuation figures for two popular cable types are listed below.

Loss per 10 feet (~3 meters) of coaxial cable length

Frequency

RG-58U

LMR-400

900 MHz

1.6 dB

0.4 dB

2.4 GHz

2.8 dB

0.7 dB

While long cable runs to an antenna create signal loss, the act of elevating the antenna another 25 feet (7.6 m) can compensate for those lost dB.

Following these 7 Murphy’s Laws of industrial wireless should help make your next wireless installation painless.

- Mike Fahrion is the chief engineer at B&B Electronics. He oversees the company’s development of next-generation industrial wireless products, including the Zlinx Xtreme IP67-rated family of radios and I/O modules. Fahrion writes the politically incorrect newsletter, eConnections.